In general, a variety of techniques have been used in the past to remove contaminants from particulate solids. For example, U.S. Pat. No. 2,965,522 describes a technique for regenerating fouled filter aids by water washing to remove the foulants. The technique involves passing the fouled filter aid through a series of hydrocyclones with separate water streams being introduced to each hydrocyclone to wash the filter aid. U.S. Pat. No. 4,288,329 and U.S. Pat. No. 4,406,796 describe a technique for cleaning mill scale with solvents to remove oil and water. With increasing emphasis on environmental concerns, techniques have been developed to remove contaminants from soil as exemplified in U.S. Pat. No. 4,606,774 and U.S. Pat. No. 4,662,948.
Particular concerns with removal of contaminants from particulate solids have arisen in the drilling of oil and gas wells wherein drill cuttings generated during drilling operations are flushed from the wellbore by drilling fluids (sometimes referred to as "drilling muds"). A serious problem exists with disposal of these drill cuttings and other residual solids after separation from the drilling mud because of oil contamination. After being separated from the drilling mud, these solids have oil on their surfaces and sometimes within their porous structure. This is particularly true when an oil-based drilling mud is employed although formation oil may also be present regardless of the type of drilling mud used. Because of the increasing environmental concerns, it is essential to have economic means to clean these oil-contaminated drill cutting solids and enable disposal in an environmentally acceptable manner. This is very significant in offshore operations where it is desirable to be able to discharge the solids overboard from the drilling rigs and avoid having to transport them to shore for disposal.
With respect to drill cuttings, a variety of techniques have been suggested for dealing with this problem.
For example, the use of surfactant solutions to wash drill cuttings to remove oil and other contamination has been suggested in U.S. Pat. No. 3,688,781, U.S. Pat. No. 3,693,733, U.S. Pat. No. 3,716,480, U.S. Pat. No. 3,860,019, U.S. Pat. No. 4,175,039, U.S. Pat. No. 4,546,783, U.S. Pat. No. 4,595,422 and U.S. Pat. No. 4,645,608. These approaches attempt to wash adsorbed oil-based mud from the surface of the drill cuttings and have been largely unsuccessful because oil-based muds are specially formulated with powerful oil wetting agents that resist the detergent action of aqueous wash solutions. Additionally, detergent-laden water, which may be even more toxic to marine organisms than the oil on the drill solids, is continuously discharged into the marine environment.
Steam stripping of the cuttings to remove oil contamination has been suggested in U.S. Pat. No. 4,209,381 and U.S. Pat. No. 4,395,338. In these techniques, steam is used to strip the more volatile oils from oily drill cuttings followed in some cases by distillation of the remaining solids to remove the higher boiling oil fractions. The methods are particularly impractical offshore because of the excessively high energy requirements to generate the quantity of steam needed and the high temperatures needed to distill the oil.
Other thermal methods involving heating of the cuttings to volatilize or incinerate the oil contamination have been suggested in U.S. Pat. No. 3,693,951, U.S. Pat. No. 4,139,462, U.S. Pat. No. 4,304,609, U.S. Pat. No. 4,411,074, U.S. Pat. No. 4,606,283, U.S. Pat. No. 4,683,963, U.S. Pat. No. 4,726,301 and EP Publication Application No. 0005273. Typically, the high temperatures required for these processes is supplied by electrical resistance heating, electrical induction heating, infra-red heaters, or high temperature heat transfer fluids. The methods have been at least partly unsuccessful for reasons already cited. The total amount of energy to heat all of the solids and boil all of the liquids off the cuttings is excessively high. Also, it is very dangerous to operate any equipment offshore in which hydrocarbon vapors are generated at temperatures well above their flash point.
Solvent washing or extraction techniques to remove oil contamination from cuttings have been suggested in U.S. Pat. No. 4,040,866, U.S. Pat. No. 4,434,028, U.S. Pat. No. 4,836,302 and PCT Published Application No. W082/01737.
In particular, U.S. Pat. No. 4,040,866 teaches the use of a mutual solvent to clean oily drill cuttings. A mutual solvent is one that is soluble in both oil and water. In this process, oily liquid is removed from the solids with a mutual solvent like ethylene glycol monobutyl ether; however, the mutual solvent and oil mixture remains on the cuttings and must be washed away with water followed by centrifuging to recover the cuttings. This method has proven impractical because two undesirable process streams are created. Large quantities of solvent (approximately equal to the original volume of oily liquid on the solids) are washed from the solids with water and discharged with the water into the environment. It is probable that the solvent is even more toxic to marine organisms than the oil which was removed from the cuttings. Additionally, large volumes of mutual solvent become contaminated with dissolved oil and must be either discarded or purified and recycled. The cost of mutual solvents prohibits simple disposal. Further, the high boiling point and high latent heat of vaporization of mutual solvents make their separation from oil by distillation difficult, expensive and hazardous.
U.S. Pat. No. 4,434,028 teaches a high pressure process for the use of a solvent which is miscible with oil but essentially immiscible with water to clean oily drill cuttings. In this process, a substance that is typically a gas at ambient temperature and pressure is compressed sufficiently to convert the gas to a liquid which then becomes a suitable solvent for the oil associated with drill cuttings. The liquified gas is then flowed, batchwise, through a vessel packed with oily solids. When the solids have been washed sufficiently clean, the chamber is depressurized allowing the solvent to flash into a vapor, leaving the solids free of oil and solvent. The oil-contaminated solvent can also be flashed to a vapor to separate it from the oil and allow it to be recycled. This process has not been successful on offshore drill sites for several possible reasons. High pressure is required to convert the normally gaseous material to a liquid so it can dissolve the oil on the solids. Mechanical problems associated with moving solids repeatedly into and out of high pressure vessels without leakage are formidable. Also, the batchwise nature of the process is not compatible with the continuous process of drilling and generation of drill cuttings. Finally, mechanical crushing of the cuttings prior to extraction requires heavy, bulky, maintenance-prone equipment which is especially undesirable for uses in offshore drilling operations.
U.S. Pat. No. 4,836,302 teaches a system for cleaning drill cuttings using a cleaning column. The system is stated to overcome many of the problems associated with the techniques described above and is contained so as to minimize solvent escaping to the atmosphere. However, the use of a cleaning column to process all the cuttings presents certain material handling problems with respect to the very fine particles of cuttings which may constitute a significant portion of the overall cuttings.
PCT Published Application No. W082/01737 describes a technique for reducing oil contamination on drill cuttings which involves washing the contaminated drill cuttings, preferably after screening to remove fines and supernatant drilling mud, with a single halogenated solvent. The resulting slurry is then macerated and processed through a single continuously running decanter centrifuge to separate the cleaned solids. While the system will reduce oil contamination on drill cuttings, a substantial volume of solvent is required to achieve significant cleaning.